Multicellular living organisms exhibit specialization of function among various tissues. For example, different cell types have evolved to handle specific functions more efficiently than unicellular organisms. See, e.g., Gilbert (1991) Developmental Biology (3d ed.), Sinauer Associates, Sunderland, Mass.; Browder et al. (1991) Developmental Biology (3d. ed.), Saunders, Philadelphia, Pa.; Russo et al. (1992) Development: The Molecular Genetic Approach, Springer-Verlag, New York, N.Y.; and Wilkins (1993) Genetic Analysis of Animal Development (2d ed.) Wiley-Liss, New York, N.Y. Each cell within a multicellular organism typically experiences peculiar environmental factors distinct from cells located elsewhere in the organism. Spatial and microenvironmental differences require different cellular functions. The evolutionary response to the different environmental effects on the cells has been for the different cells to specialize in function.
The evolutionary development of specialization in different cells is paralleled by an ontological development of cell types. The adult form is derived initially from a single or few cells. This cell divides into multiple cells and the cells differentiate into a diverse group of cells, most exhibiting some degree of specialization in location or function. The inability to modulate or steer development of cells within a developing organism prevents proper formation or development of specialized tissues and organs or prevents repair of damaged tissues, resulting, e.g., though disease or aging.
Within the early embryo, particular layers, e.g., endoderm, mesoderm, and ectoderm, are destined to differentiate to form specific organs and cell types. For example, hematopoiesis is the process by which all blood cells are formed from multipotential, undifferentiated hematopoietic stem cells (HSCs). The first active site of hematopoiesis occurs in the yolk sac, both in birds and in mammals, at approximately day 7.5 of gestation, In mammals, it has been suggested that all hematopoietic activity results from the colonization of the embryo with cells that migrate from the yolk sac to the fetal liver after the activation of circulation by day 8.5. These cells would later colonize the bone marrow and be responsible for the formation of blood cells for the entire life of the organism. HSC activity was defined in these studies as the formation of day 8 CFU-S--macroscopic colonies on the spleen of recipient mice at 8 days posttransplantation. See, McCulloch and Till (1964) Rad. Res. 22:383-396, which describes an assay later shown to detect only committed hematopoietic progenitors. Recent studies in the mouse showed that the splancnopleura, the para-aortic, or the aorta, gonada, and mesonephros region (AGM) contain hematopoietic activity earlier than the fetal liver. Moreover, complete multilineage, long-term repopulation of irradiated mice with cells derived from the AGM region was reported.
Embryonic stem (ES) cells are derived from the inner cell mass of blastocysts and appear to resemble the primitive ectoderm of the postimplantation embryo. Culture systems of ES cells that allow their differentiation in vitro into EBs containing hematopoietic activity have been described. Analysis of proteins important in these early developmental stages are likely to be important in the development of the functions of the resulting cells.
However, many of the proteins and biological activities crucial to early differentiation and physiology of these cells remain unknown. Moreover, lack of recognition genes regulated in the early development of these cell types is likely to delay recognition and definition of important functions of fundamental importance. Thus a need exists for better description of the factors and mechanisms involved in signals in differentiation and development of mammalian cell types. The present invention provides this and many other new teachings.